Bottom up vs. top-down or why the best evidence of bottom-up forcing is in Frank et al. 2005

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I'm crushed with teaching at the moment, so this post is getting less attention than it deserves, but I wanted to get this out anyway.  Since 2005, I've been involved in a scientific skirmish about the degree to which pelagic marine ecosystems are structured from the bottom-up or from the top-down.  The latest round in this debate is now available in a pair of short communications in Fisheries Oceanography (1 and 2).

The top-down view is championed by Ken Frank from BIO.  Beginning with a 2005 Science paper, he has had a series of high profile papers presenting an argument that the Eastern Scotian Shelf ecosystem has experienced a trophic cascade caused by the removal of groundfish--principally, cod.  The hallmark of a trophic cascade is a flip-flop pattern as you move down the food chain.  So, according to his view, when the cod collapsed in the late 1980s, it led to an increase in shrimp, a decrease in large zooplankton, and an increase in phytoplankton, and a decrease in nutrients.

The bottom-up side has been led by Chuck Greene from Cornell, with contributions from me and a host of others.  Our view, outlined in a series of less high profile papers (3 and 4) has been that the changes in the NW Atlantic Shelf ecosystems are ultimately tied to physical forcing.  We documented a decrease in salinity in the Gulf of Maine in the 1990s, increased phytoplankton, increased small copepods, decreased large copepods (i.e. Calanus), and increased small pelagic fish (herring).  Implicit in our view is that changes in groundfish have a limited impact on the pelagic food web. 

As with many scientific debates, the debaters (debatees?  debatants?) are not always comparing apples to apples.  As Frank points out in his note, our work has focused heavily on the Gulf of Maine while his has focused on the Eastern Scotian Shelf.  Never mind that I looked at the plankton changes in the Eastern Scotian Shelf along with several other regions along the shelf in my 2010 JPR paper--that's OK, Chuck forgets about that paper, too.  There is probably a resolution here, but that's not the point of this post.  No, the purpose of this post is to show how some of the best evidence of bottom-up forcing of NW Shelf ecosystem is in Frank's original 2005 paper. 

Frank's 2005 paper hinges on a figure that shows the trophic flip-flops.  However, there are two physical time series that he presents, but quickly dismisses.  For example, on the left are his groundfish time series plotted on top of his time series of bottom temperatures (I pulled the time series from a PDF of his paper using Illustrator):
On the right,  I've shifted the bottom temperature forward by six years.  The rise and fall of groundfish is strongly correlated with temperature.  Temperature leads fish, as you would expect from a bottom-up effect, and the six year time lag is a reasonable generation time for larger groundfish.  I have not done the statistics, but this correlation is stronger than any in his paper (that's a low bar to cross though, since no statistics appear in the paper).

But wait, there's more!  Close readers will quickly note that even if temperature was driving the changes in cod, the cod could still be driving the train.  Here is Frank's phytoplankton time series plotted on top of the stratification index that appears in his paper:
Higher phytoplankton levels are associated with increased stratification, consistent with the hypothesis Chuck and I put out in our 2007 paper.  Again, I haven't done the stats, but I'm confident that this relationship is stronger than any in the paper.  

This debate is both academically interesting and practically important.  On the practical side, understanding how fish populations influence marine food webs is vital for understanding the impact of fishing.  Understanding the impact of physical forcing is vital for understanding how these ecosystems will respond to climate change.  On the academic side, you can view this as a debate between ecology and oceanography.  Ecology has a long tradition of documenting the importance of predators--think about Paine's classic work in the intertidal or Estes' work in kelp forests.  Oceanography has tended to emphasize the connection between physics and biology.  Clearly both predation and physics are important, the real question is where, when, and why one process dominates over another.  I've always considered myself an ecologist (my Ph. D. is in Ecology and Evolutionary Biology) who happens to work in the ocean, and Chuck recently reminded me that Robert Paine was on his committee.  I'd like to think I'm open to the possibility of strong top down forcing in the ocean; however, I think the weight of evidence suggests that physics has a stronger influence on plankton than fish.

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This page contains a single entry by Andy Pershing published on November 28, 2012 4:45 PM.

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